In modern television systems it is known to automatically control the contrast and brightness of a reproduced image. For example, a TV system with automatic contrast control to inhibit "white spot blooming" is described in U.S. Pat. No. 5,003,394 entitled DYNAMIC VIDEO SYSTEM INCLUDING AUTOMATIC CONTRAST AND "WHITE STRETCH" PROCESSING SECTIONS issued to William A. Lagoni and assigned to the assignee of this application. Other TV systems which include automatic contrast control are described in the following patents pending applications: (a) U.S. Pat. No. 5,204,748 titled BEAM CURRENT LIMITING ARRANGEMENT FOR TELEVISION SYSTEM WITH PICTURE-IN-PICTURE PROVISIONS filed for William A. Lagoni; (b) Ser. No. 751,810 titled VIDEO SYSTEM INCLUDING APPARATUS FOR DEACTIVATING AN AUTOMATIC CONTROL ARRANGEMENT filed for W. A. Lagoni and R. L. O'Brien; and (c) U.S. Pat. No. 5,245,434 titled AUTOPIX CIRCUIT WITH INSERTED VERTICAL BLANKING filed for Thomas D. Gurley. The above identified applications are assigned to the assignee of this application, and their teachings as well as those of the above identified patent are incorporated herein by reference.
Automatic contrast control (which is also referred to as "autopix" where "pix" is an abbreviation for "picture") prevents loss of detail sharpness in highlight (white) areas due to blooming, while permitting high contrast (and therefore subjectively bright) images when the signal peaks remain below the blooming threshold.
Automatic contrast control circuitry used in modern TV systems is shown in FIG. 1. [For ease of illustration, only those portions of a TV system deemed pertinent to a discussion of the invention are shown in FIG. 1. A more detailed description of the TV systems is presented in the references cited above.]
Referring to FIG. 1, there is shown main and auxiliary video inputs 1 and 3, respectively, coupled to a picture-in-picture (PIP) processor 5 which is also controlled by a receiver control 7. PIP processor 5 provides signals (C and Y) to luminance circuit 9 and to chrominance circuit 11. The outputs of luminance and chrominance circuits 9 and 11, respectively, are applied to a matrix 10 whose outputs are red(r), blue(b), and green(g) color signals which are applied to respective inputs of a contrast control section 13r, 13b, and 13g. The contrast control section (e.g., 13r, 13b and 13g) is responsive to the red (r), blue (b) and green (g) color signals and its outputs are applied to a brightness control section (e.g. 15r, 15b and 15g) whose outputs are coupled via drivers (e.g. 17r, 17b and 17g) to a picture tube (e.g. 19).
The automatic contrast control arrangement includes a combiner circuit 47 for deriving a "combined" signal (e.g., SUMY) from the ouputs of the brightness section. The combined signal (i.e., SUMY) as used herein and in the claims appended hereto is representative of the luminance component of the displayed image. The combined signal (SUMY) is then processed via an autopix circuit 41 comprised of a peak detector 49 and a comparator 50 whose output is fed back via a buffer 51 to the control input terminal 13 of the contrast control section (13r, 13b, 13g) of the TV system. The autopix loop 41 comprising peak detector 49, comparator 50, and buffer 51 defines a feedback loop coupled between the output (terminal 14) of the combiner circuit 47 and the input control (terminal 13) of the contrast control section which determines the closed loop gain of the contrast control section. The open loop gain of the contrast control section (13r, 13g, 13b) is determined, in part, by a customer contrast control unit 60 which is driven by receiver control 7.
Contrast control unit 60 includes a common control microprocessor 63, a buffer amplifier 65 and a low pass filter 67. The microprocessor 63 is used to control various functions such as peaking, contrast and brightness. Under the control of microprocessor 63 and a switching element represented by N63, a pulse signal (PS) and its logical complement PSN are generated. The pulse signals PS and PSN include pulses which are pulse width modulated (PWM) to represent the customer's contrast control settings, also defined and referred to herein as customer control steps. The signal PSN (which is the logical complement of PS) is produced at node 29 of microprocessor 63 and is applied to the input of the buffer amplifier 65.
Amplifier 65 is responsive to the pulse width modulated pulse signal (PSN) and produces at its output node 75 amplified and buffered pulse width modulated output pulses (in phase with the PS signal) which are then applied to the input of a low pass filter 67. Filter 67 represented by a series-connected resistor R7 and a shunt-connected capacitor C1, filters the pulse width modulated signals produced by amplifier 65 to produce a DC user contrast control voltage (Vc) at node 670 which is applied to terminal 13.
In the arrangement shown in FIG. 1, an automatic beam current limiter 52 is also coupled to terminal 13. Consequently, capacitor C1 of low pass filter 67 is shared by circuits 60, 52 and 41 to filter and store the respective control voltages generated by user contrast control unit 60, automatic beam current limiter 52 and automatic contrast control unit 41. The control voltage (Vc) developed across capacitor C1 is thus a combination of the individual control voltages generated by control units 60, 52 and 41.
For the embodiment shown, it is assumed that increasing the direct current (DC) contrast control voltage (Vc) corresponds to increasing gain, and therefore increasing contrast, and that decreasing the DC contrast control voltage (Vc) corresponds to decreasing gain and contrast. [This corresponds to the further assumption that white-going portions of the processed luminance output signal (SUMY) are positive going.]
As shown in FIG. 1, automatic contrast control unit (autopix) 41 includes a peak detector 49 which detects the peaks of the white-going portion of the processed luminance output signal SUMY. The output voltage of white peak detector 49 is coupled to comparator 50. Peak detector 49 and comparator 50 are arranged to decrease the contrast control voltage as a function of the peak amplitude of the luminance output signal when the peak exceeds a predetermined reference voltage.
A problem with the TV system of FIG. 1 is that, for some signals, the autopix feedback loop limits the range over which the contrast control may vary. A drawback of this limitation is that since autopix is a relatively new feature, a customer/user trying to adjust the contrast may believe the TV set is defective because the contrast can not be made to vary in the accustomed manner.
The extent of the limitation may be better appreciated by reference to curve A of FIG. 2, which shows the SUMY output at terminal 14. In FIG. 2, the ordinate (y axis) represents the amplitude (in volts) of the video output voltage (SUMY) produced at terminal 14 of FIG. 1, and the abscissa (x axis) represents the 63 control steps (or settings) available to the customer/user to increase (going from step zero to step 63) the contrast. Curve A shows the VIDEO or SUMY output for the condition when the autopix loop 41 is closed; i.e., there is feedback via the feedback loop 41 from terminal 14 to contrast terminal 13.
It is seen that the VIDEO output varies linearly in response to the customer's control steps in the range from zero to 21. However, for customer control steps above 21, the response is flat. That is, the operation of the autopix feedback loop limits the user's ability to alter the contrast above control step 21 with the video output voltage being held at approximately 3.5 volts. Thus, in a system which theoretically provides for 63 control steps, only the first 21 may be effectively used.